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Applied Catalysis A, General (v.289, #1)
C-14 studies in xylene isomerization on modified HZSM-5 by Frank Bauer; Evelin Bilz; Annette Freyer (pp. 2-9).
Deposition of carbonaceous residues and tetraethoxysilane has been used to passivate the external surface of nano-sized HZSM-5. Using a real C8 aromatic feedstock, both modification methods let to a reduction of xylene loss during xylene isomerization, i.e., a decrease in undesired disproportionation reactions yielding toluene and trimethylbenzenes. The higher performance obtained for the pre-coked sample can be explained by a more efficient deactivation of strong external acid sites. The effect of surface modification on reaction mechanism was studied by using C-14-labeled toluene and ethylbenzene. At low ethylbenzene conversion, transalkylation reactions forming ethylmethylbenzene have been observed, whereas at higher conversion the additional build-up of aromatics from the ethyl group (splitted off during dealkylation) can be revealed by Radio-HPLC data. Experiments with C-14-labeled toluene confirm a supplementary bimolecular pathway of xylene isomerization via transmethylation reactions, which are assumed to take place predominantly on the external surface of medium-pore zeolite ZSM-5.
Keywords: Xylene isomerization; HZSM-5; Surface modification; Pre-coking; Silanization
Steady state isotopic transient kinetic analysis (SSITKA) of CO hydrogenation on different Co catalysts by Vidar Frøseth; Sølvi Storsæter; Øyvind Borg; Edd A. Blekkan; Magnus Rønning; Anders Holmen (pp. 10-15).
Cobalt Fischer–Tropsch catalysts with different supports, promoters, cobalt loading and surface area have been characterised under methanation conditions (483K, 1.85bar, H2/CO=10) by steady state isotopic transient kinetic analysis (SSITKA). The catalysts have previously been studied under Fischer–Tropsch conditions and they show large differences in overall activity and selectivity. However, the intrinsic site activity measured from SSITKA is essentially the same for all the catalysts. The catalysts vary in the number of available active surface sites, and the difference in overall activity can be attributed to a change in the number of surface active intermediates, either by a change in coverage or a change in total number of active surface sites. In addition, in situ CO chemisorption at 373K has been done to confirm the number of surface sites determined by H2 chemisorption. The CO chemisorption at 373K gives approximately the same number of surface sites provided that CO adsorbs by covering two adsorption sites, i.e. either dissociatively or by bridge bonding.
Keywords: Methanation; Cobalt; Alumina; Silica; Titania; SSITKA
An isotopic study of the transhydrogenation of propyne with propane over a potassium-doped chromia/alumina catalyst by S David Jackson; Isobel M. Matheson; Geoff Webb (pp. 16-21).
The transhydrogenation of propyne and propane to propene was studied over a potassium-doped chromia/alumina catalyst over the temperature range 773–873K. In the absence of propane, propyne further reduced the catalyst and produced carbon deposits and methane. Only trace quantities of propene were observed. As expected propane in the absence of propyne underwent dehydrogenation to from propene close to thermodynamic limitations. The transhydrogenation reaction was shown to require full dehydrogenation of propane to propene by use of isotopic labels. Transhydrogenation was observed at 823 and 873K. By 873K, the conversion to propene was such that 80% of the C-3 stream was propene. However, the yield of propene was low due to secondary reaction leading to methane and carbon.
Keywords: Chromia; Transhydrogenation; Propane; Propyne
Mechanistic study on the methane activation over various supported molybdenum carbide catalysts with isotopic tracer methods by Shuichi Naito; Akiko Takada; Satoshi Tokizawa; Toshihiro Miyao (pp. 22-27).
The mechanism of methane activation in CH4CO2 reaction over various unsupported and supported Mo2C catalysts was studied by means of an isotopic tracer technique employing13CH4 and CD4 as tracers. Incorporation of carbide carbons into CO took place in the case of CH4CO2 reaction over unsupported Mo2C, which was retarded significantly by dispersing Mo2C onto ZrO2 support. Methane dissociation step was also retarded significantly over supported Mo2C catalysts, and the durability of the catalysts was improved by preventing inactive carbon accumulation. By dispersing Mo2C on Al2O3, novel active acidic sites were produced, where CH4CD4 exchange reaction took place even below room temperature through an associative mechanism.
Keywords: Methane; Dry reforming; Molybdenum carbide; Alumina; Isotopic exchange
Effect of pressure on H/D exchange during n-octane conversion with Pt-SiO2 catalysts by Yuguo Wang; Burtron H. Davis (pp. 28-36).
For atmospheric conditions a hydrocarbon undergoes dehydrocyclization by a mechanism that involves nearly irreversible adsorption of the reactant. Hence, there is essentially no H/D exchange when a mixture of D2 and hydrocarbon are passed over the catalyst. The current study shows that this is not the case for conversions at higher pressures (6.8atm; 0.69MPa). At the higher pressure, there is exchange of H and D to produce a nearly statistical distribution of H and D in the unconverted reactant as well as the products. The pressure dependence is consistent with a mechanism in which the H2(D2) pressure limits the second dehydrogenation step. This mechanism accounts for the H2 dependence of both H/D exchange and the competitive conversion of a mixture of alkane and cycloalkane.
Keywords: Naphtha reforming; Dehydrocyclization; Deuterium; Isotopic tracer; n; -Octane
A tool for direct quantitative measurement of surface Brønsted acid sites of solids by H/D exchange using D2O by Guillaume Koehl; Nicolas Keller; François Garin; Valérie Keller (pp. 37-43).
A tool for direct and quantitative determination of surface Brønsted acidity of solids in terms of number, type and force based on quantitative H/D exchange kinetic between gaseous D2O and the acid solid, i.e. the hydron of the –O–H acid surface groups, has been developed. This method has been applied to materials covering the whole range of acidity, from very low acidic materials (e.g. carbonaceous solids) to high acidic materials (e.g. acidic oxides or zeolites). Calibration was performed using well-characterized zeolites with known acidities and allowed the number of surface Brønsted acid sites (in μmol/g) to be determined using the integrated surfaces of the H/D experimental exchange curve given by the direct measurement of the exchanged HDO molecule signal by mass spectrometry. The determination of the force of the acidity was directly linked to the rate constant of the first step of the H/D exchange. This method allows, after deconvolution of the H/D experimental curve, the number and force of each type of surface Brønsted acid sites to be determined.
Keywords: H/D exchange; D; 2; O; Surface brønsted acidity; Zeolite; Oxide; Carbon
Hydrogenation of olefins over hydrotreating catalysts by M. Brémaud; L. Vivier; G. Pérot; V. Harlé; C. Bouchy (pp. 44-50).
In connection with the specific requirements of the hydrotreating of FCC gasoline, the selectivity of hydrotreating catalysts in hydrodesulfurization with respect to the hydrogenation of olefins has to be controlled and if possible improved. The aim of this study was, on the one hand, to ascertain the role of Co and Ni on the activity of MoS2 catalysts in the activation of dihydrogen and in the hydrogenation of olefins and, on the other hand, to identify the origin of the hydrogen involved in the hydrogenation reaction.It was shown that the promotion effect of Ni on both the hydrogenation of cyclopentene (150°C, fixed bed reactor) and the hydrogenation of propene (80°C, recycling reactor) was higher than that of Co. This is in accordance with the fact that CoMo catalysts are more efficient in the selective hydrodesulfurization of FCC gasoline than NiMo catalysts.The hydrogenation of propene was carried out on catalyst samples, which were pre-treated with D2S at the reaction temperature (80°C) then with helium at 200°C in order to desorb part of it. Under these conditions it was shown that D-atoms coming from D2S were added to propene to form mainly mono-deuterated propane. No isotopic exchange between propane and D2 was detected under these conditions. The comparison of the initial product distributions in propane and in gas phase hydrogen indicated that D-atoms issuing from D2S were added to propene directly. This was particularly clear with the NiMo/Al2O3 catalyst and also with the non-promoted catalyst. With the CoMo/Al2O3 catalyst, the activity in isotopic exchange was so high that the complete H–D scrambling between D2S and H2 occurred even at very low hydrogenation conversion so that it was not possible to conclude. It was also found that the presence of gas phase hydrogen was necessary for the reaction to occur, which is considered further evidence in favour of the heterolytic dissociation of H2 on sulfides.
Keywords: Sulfide catalysts; Promoting effect; H–D isotopic exchange; Propene hydrogenation; D; 2; S; Hydrogen dissociation
Description of coordinatively unsaturated sites regeneration over MoS2-based HDS catalysts using35S experiments combined with computer simulations by Franck Dumeignil; Jean-Francois Paul; Edouard Veilly; Eika W. Qian; Atsushi Ishihara; Edmond Payen; Toshiaki Kabe (pp. 51-58).
By combining experimental results and computer simulations, we previously showed that the coordinatively unsaturated sites (CUS) formation over MoS2 is most likely to occur on the MoS2 metallic edge through the departure of an H2S molecule. In the present paper, we aimed at examining the H2S departure from MoS2 catalysts promoted with Co and Ni. The [35S]DBT HDS experiments results showed that over CoMoS/Al2O3 and NiMoS/Al2O3 catalysts, the activation energy of the H2S release reaction is essentially the same with respective values of 7.4 and 7.9kcalmol−1. Considering the H2S departure activation energy in the case of the non-promoted MoS2 surface (10–12kcalmol−1), this result illustrates the synergetic effect between Mo and Co or Ni in terms of CUS regeneration easiness. Further, preliminary computer simulations results showed that for S atoms bridged between Co atoms, a mechanism implying H2S departure from the metallic edge cannot be reasonably envisaged. Moreover, on the sulfur edge the H2S release activation energy is too high (∼13.5kcalmol−1) if we consider experimental results on CoMo/Al2O3 catalysts, but not incompatible with the experimental value obtained over Co/Al2O3 catalysts (ca. 10kcalmol−1), which suggests that the mechanism on the promoted catalysts differs from that on the un-promoted ones.
Keywords: HDS; DBT; MoS; 2; CoMoS; NiMoS; Activation energy; 35; S tracer experiments; Computer simulations; H; 2; S liberation
Aldol condensation of acetone over CsOH/SiO2: A mechanistic insight using isotopic labelling by A.S. Canning; S.D. Jackson; E. McLeod; E.M. Vass (pp. 59-65).
The solid base catalysed aldol condensation of acetone was performed over a CsOH/SiO2 catalyst using a H2 carrier gas. The products observed were diacetone alcohol, mesityl oxide, phorone, iso-phorone and the hydrogenated product of mesityl oxide, methyl isobutyl ketone. Deuterium tracer experiments were performed to gain an insight into the reaction mechanism. Over 10% CsOH/SiO2 deuterium exchange of acetone was very facile with conversion up to 92%. Isotope studies have revealed that the hydrogenation step from mesityl oxide to MIBK occurs though the addition of protium and not via deuterium. This is a result of the dilution of the deuterium pool on the catalyst surface due to facile H/D exchange of acetone. The formation of isophorone is proposed to occur via an aldol condensation of 4,4′-dimethyl hepta-2,6-dione and not through the internal 1,6-Michael addition of phorone. A mechanism is proposed.
Keywords: Acetone; Aldol condensation; Isotopic labelling; Solid base catalyst
Transient isotopic studies on 1-butene oxidation over a VO x-TiO2 catalyst in presence of water vapor by W.Y. Suprun; D.P. Sabde; H.-K. Schädlich; B. Kubias; H. Papp (pp. 66-73).
The catalytic oxidation of 1-butene was studied in the temperature range of 390–553K in presence and absence of O2 and water vapor. An increase in acetic acid yield was observed with increasing temperature up to a maximum at 520K. The addition of water promoted the formation of acetic acid and suppressed total oxidation reactions. Transient isotopic experiments with18O2/16O2 and H218O/H216O were carried out to investigate the role of gas oxygen and water in the formation of acetic acid. In absence of water the isotopic exchange was slow and complete at high temperatures. On the contrary, a fast but incomplete isotopic exchange in acetic acid was observed in presence of water. Isotopic transient experiments with18O2 and H218O revealed that18O from H218O was more readily incorporated into acetic acid than18O from18O2 pointing to a role of water as a reactant.
Keywords: VO; x; -TiO; 2; catalyst; Oxidation; 1-Butene; Transient isotopic studies; Role of water
Steady State Isotopic Transient Kinetic Analysis of the acrolein oxidation on Mo–V–W-mixed oxide catalysts by A. Drochner; P. Kampe; J. Kunert; J. Ott; H. Vogel (pp. 74-83).
Mixed oxide catalysts on the basis of molybdenum and vanadium are industrially important catalysts for the partial oxidation of acrolein to acrylic acid. For a better understanding of the oxygen pathways for an optimised model catalyst with the stoichiometric composition Mo7.5V2.5W0.5O x, the so-called Steady State Isotopic Transient Kinetic Analysis (SSITKA) method was employed. After switching from16O2 to18O2 the formation of18O-labelled acrolein was observed. Acrolein exchanges its carbonyl oxygen with oxygen from the solid catalyst. This exchange is also reflected in the isotopic distribution of the oxidation products (acrylic acid and combustion products). Subsequent desorption of acrolein, as well as further reaction to the oxidation products, can take place. Compared to the partial oxidation the incorporation of the isotopic tracer oxygen into the combustion by-products is much slower. This might be an indicator of an intermediate layer in the combustion pathway. To obtain activation parameters SSITKA experiments were performed over a temperature range between 322 and 391°C. Based on the observations, a macro kinetic model was adapted upon. The activation energy for the exchange of the carbonyl oxygen of acrolein was found to be approximately 30kJmol−1.
Keywords: Acrolein; Acrylic acid; Molybdenum; Vanadium; Tungsten; Mixed oxide; SSITKA; Isotopic labelling; Kinetic
Oxygen isotopic exchange: A useful tool for characterizing oxygen conducting oxides by J.-M. Bassat; M. Petitjean; J. Fouletier; C. Lalanne; G. Caboche; F. Mauvy; J.-C. Grenier (pp. 84-89).
The18O/16O isotope exchange depth profile technique (IEDP) followed by SIMS characterizations was applied to dense membranes of pure ionic conductors and mixed ionic/electronic conducting materials. It is a very useful tool since we obtain in both cases data concerning the oxygen diffusion in the bulk and the oxygen exchange at the surface of the materials. Comparisons were done, including results concerning the role of the surface with regards to the oxygen reduction reaction. Detailed experimental and analytical processes are given.
Keywords: PACS; 66.30−h (diffusion in solids); 72.60+g (mixed conductivity and conductivity transitions)SOFC; Mixed conducting oxides; Oxygen transport properties; Oxygen reduction
16O/18O isotopic exchange: A powerful tool to investigate oxygen activation on M/Ce xZr1− xO2 catalysts by Sumeya Bedrane; Claude Descorme; Daniel Duprez (pp. 90-96).
Two series of metal catalysts (Rh, Pt, Pd, Ru and Ir) supported over CeO2 and Ce0.63Zr0.36O2 were prepared. Catalysts were pretreated at 500°C (fresh) and further sintered either in H2 or in air at 700–900°C. All catalysts were characterized by H2 chemisorption at −85°C and/or by transmission electron microscopy (TEM). The16O2+18O2 homoexchange reaction was carried out in the 200–500°C temperature range. Rh, Ru and Ir showed the highest homoexchange rate R (per m2 metal) while Pd and to a lower extent Pt were less active. Sintering affected the metal performances differently: while R is higher on small Rh particles (fresh catalysts), the reverse situation could be observed for Ru and Ir where the sintered catalysts were more active than the fresh ones, especially in the case of CeO2.
Keywords: Isotopic exchange; Three-way catalysts; Oxygen activation; Rhodium; Platinum; Palladium; Ruthenium; Iridium; Ceria; Ceria–zirconia
Transient studies on the effect of oxygen on the high-temperature NO reduction by NH3 over Pt–Rh gauze by Evgenii V. Kondratenko; Javier Pérez-Ramírez (pp. 97-103).
The effect of oxygen on the activity and selectivity of the NO reduction by NH3 over Pt–Rh (95–5wt.%) alloy gauze at 1023–1073K has been investigated. To this end, the Temporal Analysis of Products (TAP) reactor in combination with isotopic tracers was applied. Single pulse experiments evidenced the rapid activation of NH3 over the catalyst surface covered by adsorbed oxygen. Contrarily, NO requires an essentially reduced surface in order to be dissociated. Adsorbed oxygen species in the O2-pretreated gauze accelerate the reaction of NH3 with NO with respect to the as-received gauze. N2 was the main reaction product and traces of N2O were comparatively formed (N2O/N2 ∼ 10−3). Pulsing of an equimolar O2–15NH3–NO mixture over the Pt–Rh gauze mainly produces15NO, while the formation of N2 is largely suppressed. The selectivity to15NO and N2O in the ternary O2–15NH3–NO system diminished upon decreasing the O2/(15NH3+NO) ratio, in favor of N2. This ratio was qualitatively varied by changing the time delay between O2 and15NH3–NO in sequential pulse experiments. Our results indicate that the NH3 oxidation by O2 to NO is much faster than the NO reduction by NH3 at similar concentrations of oxygen and nitric oxide. This explains the low production of N2 and N2O in ammonia burners within nitric acid manufacture.
Keywords: NH; 3; oxidation; NO reduction; Oxygen; Platinum; Rhodium; Gauze; Mechanism; Transient experiments; TAP reactor
On the reactivity of carbonate species on a Pt/CeO2 catalyst under various reaction atmospheres: Application of the isotopic exchange technique by Frederic C. Meunier; Daniele Tibiletti; Alexandre Goguet; David Reid; Robbie Burch (pp. 104-112).
The reactivity and stability of the carbonate species formed at the surface of a Pt/CeO2 catalyst during the reverse was-gas-shift (RWGS) reaction was investigated by in situ diffuse reflectance (DRIFT) FT-IR spectroscopy at 523K. Like most oxide materials presenting some basic character, the surface of the sample (not yet exposed to the RWGS stream) exhibited a significant concentration of carbonate species, mostly of the polydentate-type. A fraction of these carbonates was reduced by H2 at 573K, leading to the formation of Pt-bound surface carbonyls. Additional carbonates (mostly bidentate species) were formed under RWGS conditions (1% CO2+4% H2 in Ar at 523K), along with formate species and more carbonyl groups. These additional carbonates, like the polydentate species, were essentially stable under Ar and, surprisingly, under H2/Ar. On the contrary, all carbonate species were readily reacted and/or exchanged under the RWGS feed, as evidenced by steady-state isotopic transient kinetic analysis (SSITKA), and under CO2 and O2 mixtures. It is concluded that the bonding strength of the carbonates and, as a consequence, the reactivity of those depends on the nature of the feed. The possibility of ceria over-reduction is discussed as being a factor that would increase carbonate stability and result in surface poisoning. The possibility of adsorption-assisted desorption is also presented. The data reported here clearly show the need to use operando or steady-state techniques (such as SSITKA) to determine the reactivity of surface species in actual catalysis conditions, as misleading conclusions can otherwise be drawn on reaction mechanisms.
Keywords: Hydrogen production; Water gas shift; SSITKA; Carbonate; Ceria; In situ spectroscopy; Operando spectroscopy; Reaction intermediate